1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, relates to internal combustion engines having a common or shared intake port for each pair of cylinders and to a fuel injection system for precisely controlling the distribution of fuel to such shared intake ports.
2. Discussion of the Related Art
Recent years have seen a demand for adapting existing diesel and gasoline engines to burn an alternative gaseous or liquid fuel sources. Gaseous fuels such as propane or natural gas are considered by many to be superior to both diesel fuel and gasoline because they are less expensive, provide equal or greater power with equal or better mileage, and produce much lower emissions. This last benefit renders gaseous fuels particularly attractive as fuel sources because recently enacted and pending worldwide regulations may tend to prohibit the use of either gasoline or diesel fuel in many engines.
Engines may be adapted for gaseous fuel combustion by replacing the standard fuel injection system with a gaseous fuel injection system or, in the case of diesel or other compression ignition engines, by adding a gas injection system and by modifying the diesel injection system so as to supply pilot fuel to the cylinders as may be required for compression ignition. In either case, gaseous fuel injectors are mounted on or in the vicinity of the engine and operated so as to control the quantity and timing of fuel supply to each cylinder.
Most existing internal combustion engines employ either a single port or a shared port air intake system. Single port systems employ a separate intake port for each cylinder and, if adapted to burn gaseous fuel, would require a separate injector for each cylinder. Shared port systems, on the other hand, employ a common intake port for each pair of cylinders and thus, if adapted to burn a gaseous fuel, would typically require special means such as an injection pipe to assure uniform gas flow to each cylinder.
Control of injection pulse timing has been used to optimize combustion by charge stratification. For systems with isolated air intake ports, all of the fuel charge injected into a given intake port will enter the associated cylinder irrespective of the timing and duration of the injection event. However, for engines utilizing a common intake port for each pair of cylinders, accurate control of the duration and timing of each injected fuel charge can be used to assure uniform fuel charge to each cylinder without employing a separate injector for each cylinder. Precise timing of gaseous fuel injection becomes critical during operation of an engine having shared intake ports since fuel must be allocated very precisely between the two cylinders fed by each shared port. Specifically, since timing of the start of the intake strokes of adjacent cylinders fed by a shared port may be separated by only 180.degree. crank angle or less (only about 1/4 of the period between intake strokes of a single cylinder), any injected gaseous fuel which is not drawn into the first cylinder will likely disperse and be drawn into the second cylinder during the intake stroke of the second cylinder. In the worst case scenario in which the entire injection pulse for the first cylinder occurs after the end of the intake stroke of the first cylinder and in which the injection pulse for the second cylinder occurs before or during the injection stroke of the second cylinder, double the desired fuel quantity may be fed into the second cylinder. This non-uniform supply of fuel may damage or even destroy the engine.